1. Field of the Invention
The invention relates to a measuring instrument housing having a viewing window, a display panel holding fixture being provided around the viewing window, a display panel on a measuring instrument that can be inserted in the display panel holding fixture, a seal for creating a seal with resin between the display panel when inserted in the display panel holding fixture and the display panel holding fixture, a seal holding fixture provided around the viewing window in the display panel holding fixture for holding the seal, and a circumferential resin slot provided between the display panel and the display panel holding fixture.
2. Description of Related Art
Measuring instrument housings serve to accommodate measuring instruments that measure, e.g., a process variable such as the flow of a medium through a tube. A measuring instrument generally includes mechanical, electrical and electronic components and often also displaying devices, which, e.g., display the flow or status information. A viewing window is provided on the measuring instrument housing for reading displaying devices, the display elements being visible through the viewing window.
Often, a measuring instrument housing is intended to isolate the inner area containing the measuring instrument from the outer area, in order to avoid damage to the measuring instrument caused by surrounding influences. For this purpose, the viewing window can have a surrounding display panel holding fixture, in which a display panel is inserted, so that the displaying elements are visible through the viewing window and the outer space is isolated from the inner space. The seal between the display panel and the display panel holding fixture can be implemented using a ring-shaped seal, for example, with a circular cross-section, for which the display panel holding fixture around the viewing window is provided. The inserted display panel lies on the seal, so that the inner area of the measuring instrument housing and the outer area are hermetically separated from one another. The display panel can be fixed in the display panel holding fixture using outer threading, which is screwed onto threading provided in the display panel holding fixture and pushes the display panel against the seal.
If the measuring instrument is used in an atmosphere at risk of explosion and the measuring instrument is not intrinsically safe, then the measuring instrument housing can be formed pressure-proof according to standard DIN EN 60079-1. A measuring instrument is not intrinsically safe when it can ignite an explosive atmosphere. The ignition can, e.g., occur due to hot surfaces or due to energy stores that are not protected. A pressure-proof encapsulated measuring instrument housing is thus capable of withstanding the increased pressure in the inner area of the measuring instrument housing that occurs during an explosion of the explosive atmosphere, and of avoiding transfer of the explosion to the atmosphere surrounding the housing. In particular, it is essential for a pressure-proof encapsulation according to standard DIN EN 60079-1 that ignition-proof gaps are formed between the outer area and the inner area in the measuring instrument housing.
Such a gap is provided between the display panel and the display panel holding fixture, which is to be ignition-proof. One possibility for ignition-proof design of this gap is filling of the gap with a resin, referred-to as a resin slot below. Normal widths of resin slots lie in the range of 0.5 mm to 3 mm. The suitable viscosity of the resins used lies in the range of about 100 mPa·s to about 12,000 mPa·s. The resin is inserted in the resin slot and flows out in the resin slot due to the influence of gravity, wherein the seal prevents the liquid resin from leaking to the outside of the measuring instrument housing. The width of the resin slots in conjunction with the viscosities of the resins account for a merely low flow velocity of the resin in the resin slot and for the risk of entrapment of air bubbles in the resin, which can impair the ignition-proof safety. The entrapment of air bubbles in the resin can be avoided by evacuating the area before filling. However, the evacuation of the area goes hand in hand with extensive technical effort and thus accompanying costs. Visual inspection of the resin is not efficiently possible due to the given spatial relations in the measuring instrument housing. In order to still guarantee an ignition-proof design of the resin slots, a large as possible resin thickness is pursued in the resin slot. However, this goes hand in hand with the risk of contaminating the inside of the measuring instrument housing or the display panel.